Jackhammer is an id Tech 2/3 engine map editor. It currently supports Quake, Quake 2, Quake 3 Arena, Half Life, and Gunman Chronicles. The latest version includes several near features including: loading models and sprites from pak/pk3 files and reloading a model from disk without reloading the editor. Click Read More to see the whole changelog.

It comes in Windows and Linux flavors, 32/64bit. You can download it from the Jackhammer website.